Method and circuit for reducing the effect of noise pulses



Oct. 21, 1941.

M. G. NICHOLSON 2,259,532

METHOD AND cmcun FOR REDUCING THE EFFECT OF NOISE PULSES Filed April 13, 1940 BUD/0 OUTPUT Ia A A l l A A A 6 v /Za J 1, 4a #0010 our/=01- Ill/DIG OUTPUT INVENTOR. Madison GNicholson ATTORNEYS Patented Get. 21, 1941 METHODAND CIRCUIT FOR REDUCING THE EFFECT OF NOISE PULSES Madison G. Nicholson, Verona, N. assignor to National Union Radio Corporation, Newark, N. J., a corporation of New Jersey Application April 13, 1940, Serial No. 329,407

3 Claims.

My invention relates to a new and improved method and to a new and improved circuit for reducing the effect of noise pulses, such as static, interference resulting from the operation of X- ray machines and other electrical apparatus, in the circuits of radio receiving sets. a

One of the objects of my invention is to reduce or eliminate the effect of noise pulses, anterior the detector of the set.

Another object of the invention is to by-pass the noise pulse anterior the detector, either wholly or partially, by means of a suitable limiting device, as a diode or other rectifier, While maintaining said limiting device under a bias or counter-voltage which will prevent the by-passing of any portion of the carrier Wave, if it-is unmodulatecl or under normal audio modulation for Voice, music, etc.

Another object of the invention is to secure said bias by means of a suitable rectifier, such as a diode.

Another object of the invention is to provide a predetermined interval of time before the effect of a noise pulse on the bias voltageiidevice is transmitted to the limiting device. I

Another object of the invention isto provide a simple circuit, which will operate by means'of a pair of rectifiers, such as diodes, combined with suitable impedances, which are preferably resistances which have minimum induction.

Other objects of the invention will be stated in the following description and drawing'which set forth preferred embodiments of the invention, it being understood that the above'general statement of the objects of the invention is intended generally to explain the same, without limiting it in any manner. 7 v

Fig. 1 is a circuit diagram of one embodiment of the invention.

Fig. 2 is a circuit diagram of a second embodiment.

Fig. 3 is a circuit diagram of a third embodiment.

In order fully to describe the invention, preferred examples thereof are stated, giving certain values of capacities, inductances, etc. However, the invention is not limited to said preferred examples, or to any part thereof.

In Fig, 1, the member I is an electronic tube, which may be the last intermediate frequency amplifier of a superheterodyne receiving set. The invention is not limited to any particular type of set or circuit or device for receiving or detecting electric waves, save that the improved limiting means are preferably, but'not necessarily,

used anterior the audio amplifying stage or stages.

. primary coil P of a step-down transformrZ.

The other end of coil P is connected at Id to the positive terminalof a suitable source of direct current. The other end of said source is grounded. The anode supply and cathode of tube l are also connected by a condenser la whose capacity is 0.1 microfarad.

The step-down transformer 2 has an effective ratio of primary voltage to secondary voltage of one to 0.45 when the secondary coil S is loaded by the audio output diode 3. This diode 3 has a resistance load which is supplied hypotentiometer 4. This has a resistance of 0.5 megohm. Said potentiometer 4 is also used as the volume control for the audio output 5. The potentiometer 4 is by-passed at said intermediate frequency by condenser 8, whose capacity is 50 micro-microfarads.

The primary coil P has an inductance of 2 millihenries. Said coil P is tuned to resonate at said intermediate frequency by a variable trimmer condenser 1, whose capacity can be varied from 20 to micro-microfarads. Said intermediate frequency is 455 kilocycles.

The secondary coil S hasan inductance of 1 millihenry. It is tuned to resonate at said intermediate frequency of 455 kilocycles by a variable trimmer condenser 8, whose capacity can be varied from '70 to 140 micro-microfarads.

The primary and secondary coils P and S are mutually coupled so that the effective step-down ratio of the transformer is as previously stated (1 to 0.45) and this value of coupling is approximately '70% of the value for critical coupling.

The anode of tube l is connected at lb to the terminal Illa of condenser 10, whose capacity is micro-microfarads. The other terminal I 01) of condenser I0 is connected to the ground at 9a through the bias-voltage diode 9. Terminal lflb is connected to the anode of diode 9. Said diode Sand the other diode [4 are identical. These diodes 9 and M are designated in the trade as R. M. A. Type 6H4GT. Each said diode has a dynamic resistance of about one thousand ohms, under the operating conditions utilized herein, at 0.25 milliampere anode current.

The anode of diode 9 and the terminal lb of condenser ii! are connected to the ground 'at 8a through resistance ll, of 1 megohm. Ter-- minal I 0b of condenser 10 and said anode are also connected through resistance I2 of 2 megohms, to the terminal I3a of condenser I3, and to the anode of diode I4. Said condenser I3 has 0.1 microfarad capacity. The terminal I3b of condenser I3 is grounded at I3c.

The limiting diode I4 has its cathode connected to the line and to the cathode of detector diode 3, anterior the first audio stage.

The improved method and the operation of the improved circuit are as follows:

Assume that an unmodulated signal or carrier wave is applied to the primary coil P andthat this unmodulated carrier wave has a peakvoltage of volts. The effect is the same as though the terminal Illa of the condenser IIl-were connected to one terminal of an alternator, the other terminal of said alternator being grounded. This alternator would impress an, alternating voltage upon the terminal IOa. While the applied alternating voltage varies in value from minus 10 volts to plus 10 volts, current will flow;

, of resistor II is relatively small, a larger percentage of the current which flows through the condenser I0 during the discharge period thereof, will flow directiy from the ground at 9a through the resistance II to the terminal IOb. Since the value II is very large, compared to the dynamic resistance of the associated diode 9, the current through condenser I0 during the discharge period is small. The capacity of the condenser I3 is made sufficiently large so that its negative voltage will be substantially constant, thus eliminating the effect of the variation in the impressed voltage of the unmodulated signal wave or carrier wave. In the specific example which is mentioned herein, a substantially constantnegative voltage of nine (9) volts will be impressed upon the anode of the diode I4.

The coils P and S are oppositely wound, but

' they may be wound in the same direction. The

since the internal resistance of the diode 3 is pressing a charge upon the condenser 13 so that,

the terminal I3a thereof willbe at a higher volte an. t e e m a 3b thereof. The volt:-

age and charge which are thus impressed upon.

the condenser I 3 will be very small.

During the next half-cycle, when, thevoltage which is impressed uponthe condenser. II} drops from plus 10 volts to minus, 10 volts, the currentv will flow through the condenser I 0; in the, direction of the arrow 13 instead of in the direction. of the arrow ,A, This may be designated; the.

discharging period of the condenserv Ill; During this discharging period no current ,fiows. from the. ground at 9a through the diode. 3- to.'the

terminal I 01), Current willflow, from the ground; at Sathroughthe resistance II to the terminal.

IDb and through condenser II] to the. ground. Current will also flow from the, groundat I3c through the condenser I3 to the point [2a, and through the resistance I2 to the terminal IIlb.

This discharge circuit of the condenser I3, omits.-

theresistance II. At the end of the first. complete cycle of the impressed voltage of theunmodulated signal or carrier wave, a negative charge will have been impressedyupon the'condenser I3, so that the terminal I3b thereof will.

have a higher voltage than the terminal I3a- This effect is cumulative so that the required.

biasing charge will be impressed upon the. condenser I3 and upon the anode of the diode I4.-in a negligible period of time. Hence no current. can pass through diode I4 until the negative voltage which is impressedupon its cathode will:

exceedthe negativebias voltage which is thus impressed upon its anode. The value of the negative voltages which is. thus impressed upon the condenser I3 and upon the anode of the diode I4, depends upon the value of the resistance 1]. For example, if, the value of the resistance,

voltage across condenser I3 is substantially constant, as far as the frequencies which pass through transformer-2 are concerned. The variation at thisfrequency is less than 20 microvolts,

If the peak signal voltage of the unmodulated carrier wave across the primary coil P is 10 4.5 volts, Under such conditions, no current will pass through diode I 4, because a substantially constant negative counter-voltage of 9 volts is impressed upon the anode of diode I4.

The resistor I-2; and condenser I I3 act as a,

filter or smoothing means to smooth the normal audio modulation of the carrier wave, as well as thevariations in the unmo dulated carrier wave, so that the negative voltage which is impressed upon the anode of diode I4 remains substantially 9 volts, under normal audio modulation, and also when the carrier wave is unmodulated.

When the carrier wave carries audio modulation (voice, music, etc.) the maximum value of the signal voltage is normally twice that of the unmodulated carrier wave. No current will flow through diode I4' under normal modulation, because the maximum negative voltage which is then impressed upon the cathode of diodeI4'is then 9 volts. The diode I4 is then'atthe-critical stage, and current will flow through said diode if a greater negative voltage is impressed on its cathode.

If a n0ise-pulse, such as static or the like, is impressed upon the carrierwave in addition to nor mal audio modulation, the maximum negative voltage which isimpressed upon the cathode of diode I4 will exceed 9 volts. pass through diode I4, thus diminishing and-limiting the amplitude of the noise pulse which is applied to the audio output diode 3. The efiectis to by-pass the noise pulse from said diode 3. The noise to signalratiois thus materially reducedg S n h o se. s at puls s a s pplie to, the bias voltage diode 9;- there is an increase in voltage across diode; load resistor II- during ay is re r han hetim dura i n; fi he is pulse, the limiting of the noise pulse is already u ren will h n.

accomplished before the voltage acrossfilter condenser l3 has been changed from its normal value. This circuit is designed for reduction of noise pulses of durations up to 0.05 second. Most noise (static) pulses are much shorter than this, although several pulses may be grouped together within such a time duration. In this case, however, the total change in bias voltage across condenser l3 and diode I4 is small, because the average voltage during a series of noise pulses is much less than the noise peaks. Hence even with a series of noise pulses, reduction is brought about, provided there is some interval between the noise pulses which permits the signal to get through.

The invention includes by-passing some or all of the noise pulse in any manner, or neutralizing its effect, either wholly or partially, in any manner, anterior the detector. The ground connections shown may be replaced by any other type of connection.

The counter-voltage which is applied to diode I4 is not necessarily constant, although this is preferred.

In the preferred embodiment, the carrier wave is utilized to impress a biasing counter-voltage upon the diode M. The invention is not limited to this, as said biasing counter-voltage can be secured from any suitable source. If a source of said biasing counter-voltage which is independent of the carrier wave is utilized, it is not necessary to have the diode 9.

Since the cathodes of diodes 3 and M are connected to each other, the current passes from the ground at 4a, through the detector diode 3, and through diode l4, only when the limiting conditions stated herein are fulfilled.

This may .be designated for the purpose of the claims, by stating that the noise limiting means and the detector are in phase. However, the diode detector 3 may be changed in phase, by reversing its cathode and anode connections, without affecting the noise limiting action.

The detector may be of any type, and the invention is not limited to a detector diode.

The invention is not limited to a circuit which has inductive coupling. Therefore, when I use the designations primary circuit and secondary circuit in the claims, these terms include any type of coupling between the electronic tube I and the detector 3.

The resistance and inductance and capacity of the circuit can be disregarded, save as specifically stated herein, and save for the resistances and other values of the tubes of the radio receiving set.

The diodes 9 and 14 can be replaced by any suitable rectifying means.

The condenser [3 may be replaced by any suitable source of direct and substantially constant current which can deliver an adjustable voltage, and whose negative terminal is connected to the anode of diode I 4. This will make it possible to eliminate condenser l0, diode 9, and resistances H and I2. In the example given, the negative bias of said source of direct current is regulated so that a constant countervoltage of 9 volts is impressed on diode l4, thus preventing it from drawing current, unless the negative voltage which is impressed upon the cathode of diode l4 exceeds minus 9 volts. This is by no means as desirable as the preferred embodiment, but it is within the scope of the invention.

Fig. 2 shows an embodiment in which the transformer 2 is a step-up transformer. The condenser 14a of Fig. 2 has the same function and the same capacity as the condenser l3 of Fig. 1. The condenser l5 has the same function as the condenser 10 of Fig. 1, and the potentiometer 4a of Fig. 2 has the same value as the potentiometer 4 of Fig. 1. The transformer 2 of Fig. 2 may be a step-up transformer of 0.45 to 1. The value of the resistance l2a of Fig. 2 is equal to that of the resistance 12 of Fig. 1. One end of the resistance [2a is connected to the anode'of the detector diode 3a. In the embodiment of Fig. 2, the diode 3a operates as a detector and it also fulfills the function of diode 9 of Fig. 1. The diode 9a now acts to damp the noise pulses. The condenser I4 impresses the necessary countervoltage upon the diode 9a, so that said diode will not draw current until the limiting conditions above mentioned have been fulfilled. A separate explanation of the action of the circuit of Fig. 2 is therefore not required. Since the transformers of the embodiments of Figs. 1 and 2 are respectively step-down and step-up transformers, the invention is not necessarily limited to any relation between the voltages at the coils of said transformers.

Fig. 3 shows an embodiment which utilizes added selectivity. The use of the improved noise-reducing system, with the addition of selectivity between the point where the noise is to be limited and the audio detector, Will result in an additional reduction of the noise pulse or pulses. The principle is therefore entirely different from that of the well-known method of reducing noise by the use of high selectivity, but without the use of a noise limiting device.

According to my invention the tuning system Which is located anterior the noise limiter, passes a frequency band which is wider than the frequency band of the desired signal. The circuit of the radio receiver which is located after the noise limiter, passes a band width which is only equal, or only approximately equal to the bane width of the desired signal. This produces a better ratio of signal to noise.

This improvement results from two factors. First, the noise pulses have a wide frequency dis tribution, which is at least as wide as the band width of the circuit or circuits which precede the noise limiter. Secondly, the noise limiter acts to limit the radio frequency voltage, regardless of frequency distribution.

If the noise pulse has a frequency distribution through a given band width, the action of the noise limiter is to limit the voltage of the noise pulse without materially changing the frequency distribution. However, additional frequencies are generated which fall outside of the frequency band anterior the noise limited. For example, by using a circuit or circu ts whose band width is identical with the band width anterior the noise limiter, between the noise limiter and the audio detector, these additional frequencies which are generated in the limiting process are eliminated. However, if the band width of the circuit or circuits which are intermediate the noise limiter and the audio detector is narrower than the band width of the circuit or circuits which precede the noise limiter, and if said inter-- mediate band width is sufiiciently wide to pass all of the desired signal frequencies, only a part of the noise which passes the limiting device will be passed on to the audio detector. Nevertheless, all the desired signal frequencies will be passed on to the detector.

Fig. 3 is substantially identical with Fig. 1 save a C O'll1511 fig condenser"- l9; Thefpriinary coil of the transformer l6"-has* the usual condenser 11;

and the secondary coil of the tfai'isfdrmerftfi has" the usuahcondenser I8? The samc partsfim Figs. 1 and 3 are designated by the sa'm'e reference thecircuit elements in the radio 'r'deiverflip to and through thetransformer 2 niust be slibsta'n characteristic than the-ante signal hand-width} mun-ame side' bands The coupli g condensenIS -has' a sinall capacityof from-1 to 2"n'iicro'1nic'rofaradsdepending upon'tne characteristics oftiansforin it; transformer '2 of Fig-3 is a stepuown t arisfcrmer;

like the transformer 2 of Fig 1 ilh' prii r'iary and secondary coils of the-tiansfbmier: I6-' re-- spective'ly have the saine nunibefof winding s so that the tra sformer a merelytfans'mit's t e signal wave; Without changi'fig the peak voltage thereof; The respective induc'tane's' of the coils of the transform'er'f I6 depend upoh the comp ete design of the radio"rciveri-anterioi' the coupling condenser I92 How''ve'r; sufi icient orking skilled in theart; r 4 I, 1

It may be assumed that" the r 'ceiver' isfa superheterodyne' of ultr'a hig-h frequency; in} the order of 40 l'ngacycles,'- n intermediate frequency of 2' mega'cycl'e's. The band 'vvidth' anterior the coupling 'condn'ser I9 may be'200 kilocy'cl'es} and the bandwidth of'th transformer I6 isionly2pykilocycles. Si cc-thesiie'cifi'ceX'- arnble has been give rrin connection with Fig.1 refers-to an intermediate f equenc of: {155 I i s kilocycles, the values of 'th -tr'arisfoinier be changed; in the- I 3. The noise iwhich passes' :hy thenoise l iin ng; device of' Fig.5 3'will 'therforebe distributed" over 200 kilocycles: Th e additional selctivity afterthe noiselimiting fde'vice, is orily20 kilocycles; Thereforethe embodiment of Fig; 3 'further" reduces the noise pulse or energy'w 1 o f the thereof which' passes' the noise-liniiti'n'g' de ce.

Nevertheless; the-desired signal passsf' throii the circuit m itsentiretmyifit savers 011M320 1 kilocycles;which is 'n o'rfnalf The-invention whichis illustrated inFi'g: s' i r notnecessarily limited to -a"- stp-ddtvr'i trans:

former z rto a transmitter l6' of shown in saidf-figuref Accord'ifig tdmy inv ntionthat I utilize anadditional: transformer 16" and current is' bypassedout of the-"circuit; anterior 1 the" detector stage of' the' 'circuitl The detector stage 'is'bpe'ra'tedindependently of said lay-passed cur'r'e'nt. Likewise; the by-p'ass' circuit isf inde pe'nde'ritof the detector; in the sense" that the rectifying action of the detector is not controlledby' the -by-'pass' circuit.- My circ'1iit -is, therefore, wh'olly difiereritirom circuits of the type shown in U; S; Patent'N 0f- 1'94';499 in'whichthe"currents which resulted fr'omobjectionable noise pulses are maintainedinthe circuit, and said currents are caused to"afi'ect the" operation of" thfle'te'ctor, by in'ipressingwpotential bias upon thegrid thereof.

I have shown several preferred embodiments of 'n'iy invention; but it is clear thatnuineroiis changes and omissions can be rn'ad'e'in said em- I bodinir'its" vi/itliblit-dpai'tiiig 'from the spirit of theihvehtion';

I'claiIn: I I v 1. A radio-receiving circuit which-ha'sa plurality of stages, said circuit having one electrode of a by-pa'ss'diodewnn'ected 'to said=circuit at a point of connection which is subsequent "to the first stage which is anterior the detector stage of saidbirc'uitl, the current in thecir'c'uit at said point of connectionfiing th'e ni'odulated'signal current of said circuit; a biasing= capacity having a first terminal thereof connected to the anode of said by-pass diode arid-having the" second terminal thereof connected to the ground, a resistance connected i'rifshuht'with said biasing capacity between the anode" of the-by-pass'diode and" the" ground; the first terminal" of the biasing capacity being'connected to the anode of a suppleme'ntal diode whose cathode is groun'ded,-the anode of the supplemental d'iodeb'eing connected to'the circuit at a, point of said circuit in which the modulated signal current" of said circuit flows.

2 A method ofdiminishing th efiec't of a noise" piil's'e in a radi'o rece'iving' circuit which has a dt'ectofstacto" which the modulated signal current-of said circuit'is sup'pl-ied, which consists in y"passing n'i'c'idulate'zd signal current Wholly o said circuit and'anterior said detector stag', -sai'd"1nodulated signal" current being thus by pas'sjed ma single direction and wholly against a substantially constant "counte'rvoltage,

producing-and -maintainingsaid countervoltage by the-unmodulated'signal current of said circuit at a value which is substantially equal to the maximum voltage-ofthe signalcurrent under normal audio Inodiilation. v

3 A method ac ording to claim- 2, in whichany substantial change in' said countervoltage under the effect of a noisepulse, is delayed up to substantially cotseconas MADISON G. NICHOLSON. 

